With DXperience v2009 vol 2, we're extending the data shaping options available to your end-users.
When data is grouped against a date-time field, you can now use the grouping column's context menu to change group interval. Choices include: yearly, monthly or Outlook style grouping...

The same menu also allows you to invoke a dialog specifying group summaries for a column.

Once group summaries have been specified, you can sort group rows by summary values. This is done via the same context menu.
